Optical fibers are commonly coupled to one another by attaching a terminus to the stripped end of each optic fiber cable, with the tip of each optical fiber flush with the tip of a ferrule of the terminus. The tips of the ferrules and fibers are aligned within a precision alignment sleeve so the tips of the two fibers abut one another. In practice, the tips of the fibers are slightly eccentric to the axis of their ferrules, and the tips lie in planes that are not exactly perpendicular to the ferrule axes. In one example, for a single mode fiber having a mode field diameter of 9 microns (0.009 mm), the eccentricity might be 3 microns. This results in an insertion loss, which is the loss of light intensity due to the light having to pass between the tips of two abutting fibers.
The insertion loss can be reduced by rotating one terminus and its corresponding optical fiber, to different positions about the terminus axis and measuring the insertion loss at each position. It is found that the insertion loss does not change much for a rotation within about ±30° of the ideal position. Accordingly, the terminus can be rotated in steps of perhaps 60° until the optimum one of six positions is reached, and the terminus then can be fixed at that position. Since a terminus occasionally has to be removed from its housing, as to clean its tip, it is desirable that any means for fixing the orientation of the terminus enable removal of the terminus and facilitate or assure that the reinstalled terminus lies at its optimum position.